In operating an internal combustion engine, it is necessary to establish values for various control parameters and, depending upon the particular control parameter, the value of the parameter may be varied continuously in accordance with one or more operating parameters of the engine.
In a spark ignition engine, for each ignition spark, or engine fire in one of the engine cylinders, it is necessary to control the spark advance so as to produce the peak combustion pressure soon after the piston has passed the top dead center position. Because the flame speed varies with the density of the air/fuel mixture, it is generally necessary to increase the spark advance angle with decreasing cylinder filling pressure. The spark advance angle must also be increased with increasing engine speed so as to allow for extra rotation of the engine crankshaft while the air/fuel mixture burns.
Until recently, the spark advance angle was established by a mechanical device responsive to manifold depression and engine speed. Such a mechanical device establishes the spark advance angle as a simple function of engine speed and load demand as represented by the manifold depression. Careful testing of engines shows that the optimum spark advance angle is a complex function of load and speed and this function cannot be matched by a mechanical device. Modern ignition systems now use empirically derived characteristics for the spark advance angle which are stored as a look-up table in a read only memory.
These spark advance characteristics are determined by testing a number of samples of an engine and establishing an optimum spark advance angle for each load/speed point.
Although this provides a much closer match to the optimum spark advance angle than was achieved with the mechanical devices, it still does not give the engine user the best possible spark advance angle for his engine throughout its life. There are a number of reasons for this. It is not possible to test enough engines to provide good statistics and the engines available during tests are often different from production engines. Also, variations in the engine characteristics may occur due to manufacturing tolerances and from small changes in engine design. During the life of an engine, various aging effects will occur in the engine and in the sensors, actuators and electronic circuitry and these will create a mismatch between the optimum characteristics and those stored in the read only memory.
In U.S. Pat. No. 4,575,800, incorporated herein by reference, there is described an adaptive control system for controlling the spark advance in a spark ignition engine or the fuel injection timing in a compression ignition engine. In the case of the spark ignition engine, small positive and negative perturbations are superimposed on the spark advance angle in synchronization with the operating cycle of the engine. The resulting changes in engine speed are used to determine the differential or slope of engine output with respect to spark advance angle. Each slope value is examined and these values are used to provide corrections to the spark advance angle with the intention of obtaining optimum values for the spark advance angle.
In the arrangement described in this patent, two perturbation patterns are described operating respectively at engine speeds from 0-2000 RPM and 2000-4000 RPM. In the first pattern, each half period of the perturbation cycle comprises one complete engine cycle and produces a range of perturbation frequencies from 0 to 8.3 Hz in the case of a four cylinder engine. In the second pattern, each half period of the perturbation cycle comprises two complete engine cycles and produces a range of perturbation frequencies from 4.2 to 8.3 Hz.
This arrangement suffers from the disadvantage that the range of perturbation frequencies produced can be either higher or lower than the resonance of the vehicle drivetrain in different gear ratios in a typical manual transmission vehicle. Consequently, there are large variations in the phase differences between the perturbation and the resulting changes in engine speed. This makes it difficult to obtain accurate values for the slope.
Arrangements are known which overcome this problem. A perturbation waveform is generated at a frequency slightly above the highest resonant frequency of the vehicle driveline and the frequency is varied so that each cycle of the perturbation waveform comprises a whole number of engine fire periods. With this arrangement, at certain engine speeds there is a synchronization of the waveform to the engine operating cycle which results in a particular cylinder continuously experiencing more perturbations of one type compared with other cylinders. Consequently, if a particular cylinder produces a higher torque than another cylinder, this will cause a bias in the measured values of the slope.